In a gas turbine engine compression of the air is effected by an axial flow compressor. The axial flow compressor is a multi-stage unit employing alternate rows of rotating (rotor) blades and stationary (stator) vanes. The function of the rotor blades and stator vanes is to accelerate and diffuse the air until the required pressure rise is obtained.
The rotor blades and stator vanes are arranged in rows which are circumferentially disposed. Each row of rotor blades directs the air passing therethrough from the immediately upstream row of stator vanes to the immediately downstream row of stator vanes. Each row of stator vanes redirects the air at an angle to the next row of rotor blades. The preferred angle of entry of the air into each row of rotor blades depends on the speed of rotation and the velocity vector of the air approaching each row.
To optimise the air flow distribution within the axial flow compressor the rotor blades and stator vanes are designed so that the airflow enters each row at a preferred angle. They are designed so that the angle at which the airflow is incident at the leading edge of the rotor blades or stator vanes is such that the airflow does not have to turn abruptly. At the correct angle the airflow turns smoothly attaching to the surfaces of the rotor blades and stator vanes. This minimises the aerodynamic losses and enables the axial flow compressor to work efficiently.
The angle of the airflow for each row has traditionally been set by experience using correlations derived from experimental test data. The experimental test data is used to set the angle, at a nominal mid-height position on the leading edge of the rotor blades or stator vanes. The angle of the airflow at the mid-height position is set to minimise the aerodynamic losses. Since the angle at which the airflow is incident at the leading edge is only set at the mid-height position aerodynamic losses are experienced at other positions along the leading edge of the rotor blades or stator vanes radially above and below the mid-height position.